Conventional linear and branched polyester resins produced by the polycondensation of different combinations of polyols and polyacids have been widely used in the coatings industry. They have been used to coat a wide range of metallic and non-metallic substrates used in a number of different industries. These industries particularly include those in which flexible coatings are desired. Particularly suitable examples include substrates used in the packaging industry, coil coatings, and certain industrial and automotive coatings. Certain coatings, particularly in the packaging industry, must undergo extreme stresses in the course of preparation and use of the packaging containers. In addition to flexibility, packaging coatings may also need resistance to chemicals, solvents, pasteurization processes used in the packaging of beers and beverages, and may also need to withstand retort conditions commonly employed in food packaging. In the coil coating industry, the coil is unrolled, coated and re-rolled. The coating used must therefore have sufficient flexibility to withstand both the rolling process as well as the subsequent stamping or other formation process during which the coil is fabricated into the desired piece or end product; the durability of the paint on the end piece or product is also a factor. Similarly, it is often desired that coatings used in the automotive industry exhibit both flexibility and durability.
High molecular weight polyesters, which typically have good flexibility and resistance to mechanical deformation, can be made by controlling the ratio of polyol to polyacid and the extent of reaction. Such polymers, however, generally have a relatively low average functionality per chain, which limits their further use in coatings. Increasing the functionality, on the other hand, may result in a polyester having a lower molecular weight. Use of low molecular weight polyester resins in coatings can result in poor substrate adhesion, limited compatibility with other types of resins, and/or difficulty in achieving the required balance of chemical resistance and flexibility.
Polyesters having a high level of functionality without sacrificing molecular weight are therefore desired.